This invention relates to improvements in electrical circuits, especially but not exclusively which include a drive stage and a control stage for powering and controlling electrical motors such as those found in electrical power assisted steering systems. In one aspect it relates in particular to an improved drive and control circuit for an electric motor in an electric power assisted steering system.
Alternatively, the invention could be applied to any circuit arranged to switch relatively high loads such as heating elements or other power actuators.
One particular application of an electrical circuit is in the field of electric power assisted steering systems. It is now well known to provide an electric motor in a steering system in order to supply an assistance torque to a steering shaft. The motor typically comprises a number of windings connected to form one or more phases. Current is supplied to each phase by one or more switching devices connected by conductive tracks defining a high power drive circuit. Due to the high currents which can flow in the windings, the drive circuit is typically required to have a high power rating, necessitating bulky electrical conductive tracks between the windings and associated switching devices.
The operation of the switching devices is controlled by a low power control circuit. As this only needs to carry lower currents, such as digital or analogue switching signals, it can comprise narrower or smaller gauge conductive tracks between components than the drive circuit.
Finally, it is necessary to connect the electrical circuit, comprising both control and drive circuits to a source of current such as a battery. Conveniently, this can be achieved by direct connection to one or more terminals provided on the end of respective tracks of the drive circuit.
Such drive control circuits have many applications beyond electric power steering systems. Any component, such as a motor or heating element, which requires a switched current supply has similar requirements for both low current and high current flow in order to make it function.
It has been proposed in our earlier application GB 9824590.5 to provide a leadframe comprising a plurality of copper frets or tracks embedded in a plastics material to provide a conductive structure which interconnects the various electrical components of the drive circuit. These include the switching devices for directing current from the battery to the desired windings, input terminals for connecting to the battery, and output terminals for connecting the frets to the windings. The control circuit is then provided on a low power printed circuit board and this is connected to the leadframe at various points as required. Since the control circuit includes only components with a low current drain, such as control devices for providing switching signals to the switching devices, a low cost printed circuit board can be used.
To ensure reliable operation of the control circuitry and drive circuitry, it is necessary to de-couple the input terminals by providing a capacitor which connects the terminals to an earthen chassis medium. This is typically an aluminium casting and the capacitor may comprise a leaded axial ceramic capacitor. A lead at one end of the capacitor may be soldered directly to the input terminal with the other lead connected directly to the chassis via a metal insert bush and screw.
The use of leaded capacitors is, however, far from ideal. The leads of the capacitor introduce parasitic inductance which reduces the effectiveness of the capacitor to decouple RFI.
The problem could be alleviated by soldering a surface mount capacitor directly to the terminal of the leadframe. However, the mounting of such a capacitor in this manner is difficult due to its small physical size and inherent mismatch in thermal expansion co-efficient between the fretwork of the leadframe and the capacitor which could ultimately lead to damaging the capacitor.
According to a first aspect, the invention provides an electrical circuit which comprises: a drive stage comprising at least a first conducting means and a control stage comprising a second conducting means, the first conducting means comprising one or more conductive tracks or links adapted to define an electrical circuit with an input terminal extending from an end portion of at least one track for connection to a source of electrical current, the second conducting means comprising a printed circuit board defining tracks adapted to carry current to/from one or more control devices and supporting at least one track connected to a surface mounted de-coupling capacitor supported by the printed circuit board, and in which a portion of the track of the first conducting means is so constructed and arranged as to provide an electrical connection between the capacitor and the terminal.
By arranging for the track from the terminal to connect to the capacitor on the pcb, parasitic losses are minimised when coupling to the capacitor. Furthermore, by mounting the de-coupling capacitor on the same pcb as the low power control circuitry it is possible to simply connect a surface mount device which could not be connected directly to the first conducting means.
Preferably, the first conducting means comprises a lead frame comprising a plurality of conductive tracks encapsulated in a non-conducting plastic material. The leadframe may be substantially planar and the portion of track between the capacitor and the terminal may be upstanding from the leadframe to connect to the pcb.
The conducting tracks of the leadframe may have a higher power rating than the conducting tracks of the second conducting means.
Preferably, the capacitor is connected at one side to the track on the peb which is connected to the track on the leadframe. The other side of the capacitor may be connected to a further track on the pcb. This may in turn be earthed to a chassis through a conductive fastener, such as a bolt. The connection may be soldered.
It is most preferred that the terminal is defined by an end portion of the track in the leadframe. The leadframe may comprise tracks encapsulated in plastic which may be so constructed and arranged as to form a non-conductive shroud around the terminal.
The portion of the track adjacent the terminal may be severed and each severed end displaced out of the plane of the leadframe towards the second conducting means. These ends may pass through openings in the second conducting means to contact the conducting track for the capacitor. They may then be soldered to the track to electrically connect to the capacitor.
In an alternative, instead of severing the track it may be continuous with an arch formed in the track. The top of the arched portion of the track may pass through the pcb to connect to the capacitor. Thus, the capacitor connects to the terminal down one side of the arch. Other spatial arrangements of the track are also envisaged which fall within the scope of the invention.
In accordance with a second aspect, the invention provides an electrical power assisted steering system including an electric motor and a drive and control circuit for the electric motor in accordance with the first aspect of the invention.
According to a third aspect, the invention provides an electrical circuit which comprises a low current stage and a high current stage, the low current stage comprising at least a second conducting means, the first conducting means comprising one or more conductive tracks or links adapted to define an electrical circuit with an input terminal extending from an end portion of at least one track for connection to a source of electrical current, the second conducting means comprising a printed circuit board defining tracks adapted to carry current to/from one or more control devices and supporting at least one track connected to a surface mounted de-coupling capacitor supported by the printed circuit board, and in which a portion of the track of the first conducting means is so constructed and arranged as to provide an electrical connection between the capacitor and the terminal.